1. Field of the Invention
The present invention relates to a step-down circuit and, more particularly, to a step-down circuit that generates an internal power supply voltage by stepping down an external power supply voltage.
2. Description of the Related Art
A step-down circuit that generates an internal power supply voltage by stepping down an external power supply voltage is known. This step-down circuit comprises an output transistor connected between an external power supply and a load circuit that receives the internal power supply voltage, and a circuit for setting the gate voltage of this output transistor.
As the micropatterning of semiconductor products advances, it is becoming necessary to step down the power supply voltage in order to secure the reliability of devices. When an external power supply voltage and internal power supply voltage are stepped down, the potential difference between the two power supply voltages decreases. This decreases a drain-to-source voltage Vds of the output transistor included in the step-down circuit, and reduces a load current flowing through the load circuit that receives the internal power supply voltage. Accordingly, a metal oxide semiconductor (MOS) transistor having high current supply capability is generally required as the output transistor.
The gate voltage of the output transistor is always held constant regardless of the load current. Therefore, the internal power supply voltage fluctuates in accordance with the load current. Although the fluctuation amount of the load current changes in accordance with the specifications of the product, the fluctuation is produced because the internal circuit operations of the device in operation modes roughly classified into a data write mode, a data read mode, and another function mode are largely different. The fluctuation in internal power supply voltage makes the circuit operation unstable, and affects the operation timings and current specifications. This problem cannot be ignored any longer if stepping down the voltage and raising the speed of a device further advance in the future.
As a related technique of this kind, a technique is disclosed which changes the size of an output transistor for applying a voltage to a load circuit, thereby changing the current supply capability of the output transistor in accordance with a load current (Jpn. Pat. Appln. KOKAI Publication No. 2005-107948).